Plasma Proteins- Chemistry, Functions and Clinical Significance PDF

Plasma Proteins- Chemistry, Functions and Clinical Significance 11/24/2024 Biochemistry For Medics 1 Plasma proteins- Introduction Plasma consists of water, electrolytes, metabolites, nutrients, proteins, and hormones. The concentration of total protein in human plasma is approxim...

Plasma Proteins- Chemistry, Functions and Clinical Significance 11/24/2024 Biochemistry For Medics 1 Plasma proteins- Introduction Plasma consists of water, electrolytes, metabolites, nutrients, proteins, and hormones. The concentration of total protein in human plasma is approximately 6.0–8.0 g/dL and comprises the major part of the solids of the plasma. The proteins of the plasma are a complex mixture that includes not only simple proteins but also conjugated proteins such as glycoproteins and various types of lipoproteins. 11/24/2024 Biochemistry For Medics 2  Components of Plasma 11/24/2024 Biochemistry For Medics 3 Separation of Plasma proteins Salting-out methods-three major groups—fibrinogen, albumin, and globulins—by the use of varying concentrations of sodium or ammonium sulfate. Electrophoresis- five major fractions Albumin α1 and α2 globulins β globulins  γ globulins 11/24/2024 Biochemistry For Medics 4 Separation of Plasma proteins by Electrophoresis 11/24/2024 Biochemistry For Medics 5 Albumin Albumin (69 kDa) is the major protein of human plasma (3.4–4.7 g/dL) Makes up approximately 60% of the total plasma protein. About 40% of albumin is present in the plasma, and the other 60% is present in the extracellular space. Half life of albumin is about 20 days. Migrates fastest in electrophoresis at alkaline pH and precipitates last in salting out methods 11/24/2024 Biochemistry For Medics 6 Synthesis of Albumin The liver produces about 12 g of albumin per day, representing about 25% of total hepatic protein synthesis and half its secreted protein. Albumin is initially synthesized as a preproprotein Its signal peptide is removed as it passes into the cisternae of the rough endoplasmic reticulum, and a hexapeptide at the resulting amino terminal is subsequently cleaved off farther along the secretory pathway. 11/24/2024 Biochemistry For Medics 7 Structure of Albumin Mature human albumin consists of one polypeptide chain of 585 amino acids and contains 17 disulfide bonds It has an ellipsoidal shape, which means that it does not increase the viscosity of the plasma as much as an elongated molecule such as fibrinogen does. Has a relatively low molecular mass about 69 kDa Has an iso-electric pH of 4.7 11/24/2024 Biochemistry For Medics 8 Functions of Albumin  Colloidal osmotic Pressure-albumin is responsible for 75–80% of the osmotic pressure of human plasma due to its low molecular weight and large concentration It plays a predominant role in maintaining blood volume and body fluid distribution. Hypoalbuminemia leads to retention of fluid in the tissue spaces(Edema) 11/24/2024 Biochemistry For Medics 9  Functions of Albumin Transport function-albumin has an ability to bind various ligands, thus acts as a transporter for various molecules. These include-  free fatty acids (FFA), calcium, certain steroid hormones, bilirubin, copper A variety of drugs, including sulfonamides, penicillin G, dicoumarol, phenytoin and aspirin, are also bound to albumin 11/24/2024 Biochemistry For Medics 10 Functions of Albumin Nutritive Function Albumin serves as a source of amino acids for tissue protein synthesis to a limited extent, particularly in nutritional deprivation of amino acids.  Buffering Function-Among the plasma proteins, albumin has the maximum buffering capacity due to its high concentration and the presence of large number of histidine residues, which contribute maximally towards maintenance of acid base balance. Viscosity- Exerts low viscosity 11/24/2024 Biochemistry For Medics 11 Clinical significance of Albumin Blood brain barrier- Albumin- free fatty acid complex can not cross the blood brain barrier, hence fatty acids can not be utilized by the brain. Loosely bound bilirubin to albumin can be easily replaced by drugs like aspirin In new born if such drugs are given, the released bilirubin gets deposited in brain causing Kernicterus. 11/24/2024 Biochemistry For Medics 12 Clinical significance of Albumin Protein bound calcium  Calcium level is lowered in conditions of Hypo- Albuminemia  Serum total calcium may be decreased  Ionic calcium remains same Tetany does not occur Calcium is lowered by 0.8 mg/dl for a fall of 1g/dl of albumin 11/24/2024 Biochemistry For Medics 13 Clinical significance of Albumin Drug interactions— Two drugs having same affinity for albumin when administered together, can compete for available binding sites with consequent displacement of other drug, resulting in clinically significant drug interactions. Example-Phenytoin, dicoumarol interactions 11/24/2024 Biochemistry For Medics 14 Clinical significance of Albumin Edema- Hypoalbuminemia results in fluid retention in the tissue spaces Normal level- 3.5-5 G/dl Hypoalbuminemia- lowered level is seen in the following conditions- Cirrhosis of liver Malnutrition Nephrotic syndrome Burns Malabsorption Analbuminemia- congenital disorder Hyperalbuminemia- In conditions of fluid depletion (Haemoconcentration) 11/24/2024 Biochemistry For Medics 15 Globulins Globulins are separated by half saturation with ammonium sulphate Molecular weight ranges from 90,000 to 13,00,000 By electrophoresis globulins can be separated in to – α1-globulins α2-globulins β-globulins  Y-globulins 11/24/2024 Biochemistry For Medics 16 Synthesis of Globulins α and β globulins are synthesized in the liver. Y globulins are synthesized in plasma cells and B-cells of lymphoid tissues (Reticulo- endothelial system) Synthesis of Y globulins is increased in chronic infections, chronic liver diseases, auto immune diseases, leukemias, lymphomas and various other malignancies. 11/24/2024 Biochemistry For Medics 17 α- Globulins  They are glycoproteins Based on electrophoretic mobility , they are sub classified in to α1 and α2 globulins α1 globulins Examples- α1antitrypsin Orosomucoid (α1 acid glycoprotein) α1-fetoprotein (AFP) 11/24/2024 Biochemistry For Medics 18 α1 globulins α1-antitrypsin Also called α1-antiprotease It is a single-chain protein of 394 amino acids, contains three oligosaccharide chains  It is the major component (> 90%) of the α 1 fraction of human plasma.  It is synthesized by hepatocytes and macrophages and is the principal serine protease inhibitor of human plasma.  It inhibits trypsin, elastase, and certain other proteases by forming complexes with them. 11/24/2024 Biochemistry For Medics 19 Polymorphic forms of α1-antitrypsin At least 75 polymorphic forms occur, many of which can be separated by electrophoresis The major genotype is MM, and its phenotypic product is PiM A deficiency of this protein has a role in certain cases (approximately 5%) of emphysema. This occurs mainly in subjects with the ZZ genotype, who synthesize PiZ, and also in PiSZ heterozygotes, both of whom secrete considerably less protein than PiMM individuals. 11/24/2024 Biochemistry For Medics 20  Clinical consequences of α1-antitrypsin deficiency Emphysema- Normally antitrypsin protects the lung tissue from proteases(active elastase) released from macrophages Forms a complex with protease and inactivates it. In its deficiency, the active elastase destroys the lung tissue by proteolysis. 11/24/2024 Biochemistry For Medics 21 Clinical consequences of α1-antitrypsin deficiency Smoking and Emphysema-A methionine (residue 358) of α1-antitrypsin is involved in its binding to proteases. Smoking oxidizes this methionine to methionine sulfoxide and thus inactivates it. Affected molecules of α1-antitrypsin no longer neutralize proteases. This is particularly devastating in patients (eg, PiZZ phenotype) who already have low levels of α1-antitrypsin. The further diminution in α 1-antitrypsin brought about by smoking results in increased proteolytic destruction of lung tissue, accelerating the development of emphysema. 11/24/2024 Biochemistry For Medics 22 Clinical consequences of α1- antitrypsin deficiency 11/24/2024 Biochemistry For Medics 23  Clinical consequences of α1-antitrypsin deficiency Cirrhosis of Liver- In this condition, molecules of the ZZ phenotype accumulate and aggregate in the cisternae of the endoplasmic reticulum of hepatocytes. Aggregation is due to formation of polymers of mutant α 1-antitrypsin, the polymers forming via a strong interaction between a specific loop in one molecule and a prominent -pleated sheet in another (loop-sheet polymerization).  By mechanisms that are not understood, hepatitis results with consequent cirrhosis (accumulation of massive amounts of collagen, resulting in fibrosis). 11/24/2024 Biochemistry For Medics 24 Clinical consequences of α1-antitrypsin deficiency 11/24/2024 Biochemistry For Medics 25 Orosomucoid /α1-acid glycoprotein  Concentration in plasma- 0.6 to 1.4 G/dl Carbohydrate content 41% Marker of acute inflammation Acts as a transporter of progesterone Transports carbohydrates to the site of tissue injury  Concentration increases in inflammatory diseases, cirrhosis of liver and in malignant conditions Concentration decreases in liver diseases, malnutrition and in nephrotic syndrome 11/24/2024 Biochemistry For Medics 26 α1-fetoprotein (AFP) Present in high concentration in fetal blood during mid pregnancy Normal concentration in healthy adult- < 1µg/100ml Level increases during pregnancy Clinically considered a tumor marker for the diagnosis of hepatocellular carcinoma or teratoblastomas. 11/24/2024 Biochemistry For Medics 27  α2-globulins Clinically important α2-globulins are- Haptoglobin Ceruloplasmin α2- macroglobulins 11/24/2024 Biochemistry For Medics 28 Haptoglobin-(Hp) It is a plasma glycoprotein that binds extracorpuscular hemoglobin (Hb) in a tight noncovalent complex (Hb-Hp). The amount of Haptoglobin in human plasma ranges from 40 mg to 180 mg of hemoglobin-binding capacity per deciliter. The function of Hp is to prevent loss of free hemoglobin into the kidney. This conserves the valuable iron present in hemoglobin, which would otherwise be lost to the body. 11/24/2024 Biochemistry For Medics 29  Haptoglobin (Contd.)- The molecular mass of hemoglobin is approximately 65 kDa Hb-Hp complex has a molecular mass of approximately 155 kDa.  Free hemoglobin passes through the glomerulus of the kidney, enters the tubules, and tends to precipitate therein (as can happen after a massive incompatible blood transfusion, when the capacity of haptoglobin to bind hemoglobin is grossly exceeded). However, the Hb-Hp complex is too large to pass through the glomerulus. Thus Hp helps to conserve iron. 11/24/2024 Biochemistry For Medics 30 Clinical Significance of Haptoglobin Concentration rises in inflammatory conditions Concentration decreases hemolytic anemias Half-life of haptoglobin is approximately 5 days, the half-life of the Hb-Hp complex is about 90 minutes, the complex being rapidly removed from plasma by hepatocytes. Thus, when haptoglobin is bound to hemoglobin, it is cleared from the plasma about 80 times faster than normally. The level of haptoglobin falls rapidly in hemolytic anemias. Free Hp level or Hp binding capacity depicts the degree of intravascular hemolysis. 11/24/2024 Biochemistry For Medics 31 Ceruloplasmin α2-globulin  Copper containing  Glycoprotein with enzyme activities It has a blue color because of its high copper content Carries 90% of the copper present in plasma. Each molecule of ceruloplasmin binds six atoms of copper very tightly, so that the copper is not readily exchangeable. 11/24/2024 Biochemistry For Medics 32 Ceruloplasmin Normal plasma concentration approximately 30mg/dL  Enzyme activities are Ferroxidase, copper oxidase and Histaminase. Synthesized in liver in the form of apo ceruloplasmin, when copper atoms get attached it becomes Ceruloplasmin.  Although carries 90% of the copper present in plasma. but it binds copper very tightly, so that the copper is not readily exchangeable. Albumin carries the other 10% of the plasma copper but binds the metal less tightly than does ceruloplasmin. Albumin thus donates its copper to tissues more readily than ceruloplasmin and appears to be more important than ceruloplasmin in copper transport in the human body. 11/24/2024 Biochemistry For Medics 33 Clinical Significance of Ceruloplasmin Normal level- 25-50 mg/dl Low levels of ceruloplasmin are found in Wilson disease (hepatolenticular degeneration), a disease due to abnormal metabolism of copper. The amount of ceruloplasmin in plasma is also decreased in liver diseases, mal nutrition and nephrotic syndrome. 11/24/2024 Biochemistry For Medics 34 α2- Macroglobulin (AMG) Major component of α2 proteins Comprises 8–10% of the total plasma protein in humans. Tetrameric protein with molecular weight of 725,000. Synthesized by hepatocytes and macrophages Inactivates all the proteases and thus is an important in vivo anticoagulant. Carrier of many growth factors Normal serum level-130-300 mg/dl Concentration is markedly increased in nephrotic syndrome, since other proteins are lost through urine in this condition. 11/24/2024 Biochemistry For Medics 35 β Globulins β Globulins of clinical importance are –  Transferrin  C-reactive protein Haemopexin Complement C1q β Lipoprotein(LDL) 11/24/2024 Biochemistry For Medics 36 Transferrin Transferrin (Tf) is a β -globulin with a 1 molecular mass of approximately 76 kDa.  It is a glycoprotein and is synthesized in the liver. About 20 polymorphic forms of transferrin have been found. It plays a central role in the body's metabolism of iron because it transports iron (2 mol of Fe3+ per mole of Tf) in the circulation to sites where iron is required, eg, from the gut to the bone marrow and other organs. Approximately 200 billion red blood cells (about 20 mL) are catabolized per day, releasing about 25 mg of iron into the body— most of which is transported by transferrin. 11/24/2024 Biochemistry For Medics 37 Transferrin Receptors There are receptors (TfR1 and TfR2) on the surfaces of many cells for transferrin. It binds to these receptors and is internalized by receptor-mediated endocytosis. The acid pH inside the lysosome causes the iron to dissociate from the protein. The dissociated iron leaves the endosome via DMT1 to enter the cytoplasm. ApoTf is not degraded within the lysosome. Instead, it remains associated with its receptor, returns to the plasma membrane, dissociates from its receptor, reenters the plasma, picks up more iron, and again delivers the iron to needy cells. Normally, the iron bound to Tf turns over 10–20 times a day. 11/24/2024 Biochemistry For Medics 38 Transferrin Receptors 11/24/2024 Biochemistry For Medics 39 Clinical Significance of Transferrin The concentration of transferrin in plasma is approximately 300 mg/dL. This amount of transferrin can bind 300 g of iron per deciliter, so that this represents the total iron-binding capacity of plasma. However, the protein is normally only one- third saturated with iron. In iron deficiency anemia, the protein is even less saturated with iron, whereas in conditions of storage of excess iron in the body (eg, hemochromatosis) the saturation with iron is much greater than one-third. 11/24/2024 Biochemistry For Medics 40 Clinical Significance of Transferrin Increased levels are seen in iron deficiency anemia and in last months of pregnancy  Decreased levels are seen in- Protein energy malnutrition Cirrhosis of liver Nephrotic syndrome  Trauma Acute myocardial infarction Malignancies Wasting diseases 11/24/2024 Biochemistry For Medics 41 C- reactive protein(β Globulin) So named because it reacts with C- polysaccharide of capsule of pneumococci Molecular weight of 115-140 kD Synthesized in liver Can stimulate complement activity and macrophages Acute phase protein- Concentration rises in inflammatory conditions Clinically important marker to predict the risk of coronary heart disease 11/24/2024 Biochemistry For Medics 42 Haemopexin(β Globulin)  Molecular weight 57,000-80,000 Normal level in adults-0.5 to 1.0 gm/L Low level at birth, reaches adult value within first year of life Synthesized in liver Function is to bind haem formed from breakdown of Hb and other haemoproteins Low level- found in hemolytic disorders, at birth and drug induced High level- pregnancy, diabetes mellitus, malignancies and Duchenne muscular dystrophy 11/24/2024 Biochemistry For Medics 43 Complement C1q (β Globulin) First complement factor to bind antibody Binding takes place at the Fc region of IgG or Ig M Binding triggers the classical complement pathway Thermo labile, destroyed by heating Normal level – 0.15 gm/L Molecular weight-400,000 Can bind heparin and bivalent ions Decreased level is used as an indicator of circulating Ag –Ab complex.  High levels are found in chronic infections 11/24/2024 Biochemistry For Medics 44 Gamma Globulins  They are immunoglobulins with antibody activity They occupy the gamma region on electrophoresis Immunoglobulins play a key role in the defense mechanisms of the body There are five types of immunoglobulins IgG, IgA, IgM, IgD, and IgE. 11/24/2024 Biochemistry For Medics 45 Different Classes of Immunoglobulins 11/24/2024 Biochemistry For Medics 46  Major functions of immunoglobulins Immunoglobulin Major Functions IgG Main antibody in the secondary response. Opsonizes bacteria, Fixes complement, neutralizes bacterial toxins and viruses and crosses the placenta. Secretory IgA prevents attachment of bacteria and viruses to mucous membranes. Does not fix IgA complement. Produced in the primary response to an antigen. Fixes complement. Does not cross the placenta. IgM Antigen receptor on the surface of B cells. IgD Uncertain. Found on the surface of many B cells as well as in serum. IgE Mediates immediate hypersensitivity Defends 11/24/2024 against Biochemistry For Medics worm infections. Does not fix complement. 47  Fibrinogen Also called clotting factor1 Constitutes 4-6% of total protein Precipitated with 1/5 th saturation with ammonium sulphate Large asymmetric molecule Highly elongated with axial ratio of 20:1 Imparts maximum viscosity to blood Synthesized in liver Made up of 6 polypeptide chains Chains are linked together by S-S linkages Amino terminal end is highly negative due to the presence of glutamic acid Negative charge contributes to its solubility in plasma and prevents aggregation due to electrostatic repulsions between the fibrinogen molecules. 11/24/2024 Biochemistry For Medics 48  Transport proteins Name Compounds transported Albumin Fatty acids, bilirubin, hormones, calcium, heavy metals, drugs etc. Prealbumin-(Transthyretin) Steroid hormones thyroxin, Retinol Retinol binding protein Retinol (Vitamin A) Thyroxin binding protein(TBG) Thyroxin Transcortin(Cortisol binding protein) Cortisol and corticosteroids Haptoglobin Hemoglobin Hemopexin Free haem Transferrin Iron HDL(High density lipoprotein) Cholesterol (Tissues to liver) LDL(Low density lipoprotein) Cholesterol(Liver to tissues) 11/24/2024 Biochemistry For Medics 49 Acute phase proteins The levels of certain proteins may increase in blood in response inflammatory and neoplastic conditions, these are called Acute phase proteins. Examples- C- reactive proteins Ceruloplasmin Alpha -1 antitrypsin Alpha 2 macroglobulins Alpha-1 acid glycoprotein 11/24/2024 Biochemistry For Medics 50 Negative acute phase protein The levels of certain proteins are decreased in blood in response to certain inflammatory processes. Examples- Albumin Transthyretin Retinol binding protein Transferrin 11/24/2024 Biochemistry For Medics 51 Abnormal Proteins 1) Bence – Jone’s proteins Abnormal proteins- monoclonal light chains Present in the urine of a patient suffering from multiple myeloma (50% of patients) Molecular weight 45,000 Identified by heat coagulation test Best detected by zone electrophoresis and immunoelectrophoresis 2)Cryoglobulins These proteins coagulate when serum is cooled to very low temperature Commonly monoclonal IgG or IgM or both Increased in rheumatoid arthritis, multiple myeloma, lymphocytic leukemia, lymphosarcoma and systemic lupus erythematosus 11/24/2024 Biochemistry For Medics 52 Functions of plasma proteins Nutritive Fluid exchange Buffering Binding and transport Enzymes Hormones Blood coagulation Viscosity Defense Reserve proteins Tumor markers Antiproteases 11/24/2024 Biochemistry For Medics 53 Clinical Significance of Plasma proteins Hyperproteinemia- Levels higher than 8.0gm/dl Causes-  Hemoconcentration- due to dehydration, albumin and globulin both are increased Albumin to Globulin ratio remains same. Causes- Excessive vomiting Diarrhea Diabetes Insipidus Pyloric stenosis or obstruction Diuresis Intestinal obstruction 11/24/2024 Biochemistry For Medics 54 Hypergammaglobulinemia 1)Polyclonal-  Chronic infections  Chronic liver diseases  Sarcoidosis  Auto immune diseases 2) Monoclonal  Multiple myeloma  Macroglobulinaemia  Lymphosarcoma  Leukemia  Hodgkin’s disease 11/24/2024 Biochemistry For Medics 55 Hypoproteinemia Decease in total protein concentration Hemodilution- Both Albumin and globulins are decreased, A:G ratio remains same, as in water intoxication  Hypoalbuminemia- low level of Albumin in plasma Causes- Nephrotic syndrome Protein losing enteropathy Severe liver diseases Mal nutrition or malabsorption Extensive skin burns Pregnancy  Malignancy 11/24/2024 Biochemistry For Medics 56 Hypogammaglobulinemia Losses from body- same as albumin- through urine, GIT or skin Decreased synthesis Transient neonatal  Primary genetic deficiency Secondary – drug induced (Corticosteroid therapy), uremia, hematological disorders AIDS(Acquired Immuno deficiency syndrome) 11/24/2024 Biochemistry For Medics 57